Mobile devices such as mobile telephones, and in particular smartphones, are being equipped more and more with relatively high-power audio amplifiers to drive hands free speakers and provide high-quality audio functionality. One implementation uses a class-AB amplifier as the audio amplifier. However, the efficiency of such an amplifier does not generally exceed 20-25% in most practical situations, leading to high current consumption, which is undesirable. Class-D amplifiers offer an alternative approach with a significantly higher efficiency than class-AB amplifiers.
One difficulty is that the micro speakers that are generally used in mobile devices are relatively fragile and may be easily damaged. A typical micro speaker may generally handle less than half a watt of continuous power. Indeed, micro speakers generally suffer from two main operating limitations.
First, there is a limit in the extent of excursion or movement of the speaker membrane that can occur without damaging the membrane. For a typical micro speaker, the maximum allowable excursion is around 0.4 mm. However, speakers generally have a mechanical resonance frequency at around 1 kHz, and at this frequency, the excursion limit of the speaker may be exceeded by a relatively low power signal. While a high-pass filter may be used to attenuate the signal energy at and below this resonance frequency, this would have a detrimental effect on sound quality. Furthermore, the mechanical resonance frequency may change significantly under varying operating conditions, e.g., depending on temperature, age and the external forces such as the blocking of the speaker port, meaning that a filter would have to suppress a relatively large frequency band.
Secondly, overheating can damage the speaker. Overheating occurs when more power is supplied to the speaker than can be dissipated. If, for example, air movement around the speaker is restricted by a blocked speaker port, cooling of the speaker becomes less efficient, and overheating may occur at relatively low power within a few seconds.
Existing approaches for protecting micro speakers from damage tend to be inadequate and/or complex. There is thus a need in the art for an improved speaker protection system and method.